The low-loss conversion of electrical energy by means of suitable power-electronics converters is gaining increasing significance, even for high power levels. Important application areas include high-voltage direct-current transmission (HVDC) and large-scale drives with electronic speed/torque control. In terms of circuitry,—since the development of highly suitable, switchable power semiconductor switches—the most widely established are V converters that are known in the (English-language) literature as Voltage Source Converters.
A particularly advantageous variant is disclosed in DE 1010 3031 A1 and designated as a modular multi-level converter. FIG. 1 shows the structure of a modular multilevel converter 1 from the prior art, taken from DE 10 2009 057 288 A1. The converter 1 has power semiconductor valves 2 or branches, that are connected to each other in a bridge circuit. Each of the power semiconductor valves 2 extends between an AC voltage connection L1, L2, L3 and a DC voltage connection 31, 32, 33 or 41, 42, 43. The DC voltage connections 31, 32, 33 are connectable via a positive pole connection 5 to a positive pole and via a negative pole connection 6 to a negative pole of a DC voltage network (not shown).
The AC voltage connections L1, L2, L3 are in each case connected to the secondary winding of a transformer, whose primary winding is connected to an AC voltage network (also not shown). For each phase of the AC voltage network an AC voltage connection L1, L2, L3 is provided. In the designated prior art the converter 1 is part of a high-voltage DC transmission system and is used for connecting AC voltage networks in order to transmit high electrical power levels between them. Such a converter 1 can also be part of a so-called FACTS system, which is used for network stabilization or for ensuring a desired voltage quality, or can be used in drive technology. The switches in the individual submodules 7 can be controlled by a control device 60. For the sake of the clarity, the control lines between the control unit 60 and the individual submodules are not shown in FIG. 1.
As can also be seen from FIG. 1, the multi-level converter 1 is constructed in a strictly modular fashion from submodules 7, which essentially have two power connections or interfaces to the outside. Different variants of the internal circuitry of such submodules 7 are known in the prior art. The simplest circuit variant is the so-called half-bridge submodule, which is already known from the above-mentioned patent document DE 1010 3031 A1 and is shown in FIG. 2. Such a half-bridge submodule has the smallest power loss for a given power and like semiconductor expenditure. The minimization of the power loss in the high-power range is of great economic and technical importance.
Instead of a half-bridge module, a full-bridge-submodule can also be used in modular multilevel converters, as is shown in FIGS. 3 to 5 in different switching states. As can be seen in FIGS. 3 to 5, the full-bridge submodule contains four semiconductor switches 10, 30, 10a and 30a, each of which has a reversed free-wheeling diode connected to it in parallel. The switches 10 and 30 form a first half-bridge and the switches 10a and 30a form a second half-bridge. Each full-bridge submodule has a first and a second terminal X1 and X2 respectively. The term “terminal” is intended in a purely functional sense and does not mean that the submodule necessarily actually comprises removable connections or clamps. This means that even if a plurality of modules is permanently connected, the inputs and/or outputs of the individual modules in this disclosure are designated as “terminals”. The full-bridge submodule also comprises a capacitor as an energy storage device.
FIG. 3 shows a case in which the voltage Ux between the second and the first terminal X2, X1 is zero (UX=0) and a positive terminal current IX>0 (technical current direction) flows.
FIG. 4 shows a switching state of the same submodule which has been activated with a positive supply voltage UX>0, which—apart from the small forward voltage of the semiconductors—is equal to the positive voltage of the capacitor.
FIG. 5, like FIG. 3, shows a state with a negligible voltage between the terminals (UX=0), but which is implemented by an alternative switching state of the semiconductor switches. Both in the switching state of FIG. 3, and in the switching state of FIG. 5, a current iX can flow through the submodule without the capacitor 9 absorbing (i.e. receiving) or delivering energy. An advantageous control method for this state could provide for the switching states according to FIG. 3 and FIG. 5 being activated alternately, in order to distribute the resulting transmission power loss over all four semiconductor switches 10, 10a, 30, 30a. This can certainly reduce the average semiconductor temperature, but does not achieve a significant reduction in the power loss of the submodule as a whole. The transmission voltage of the submodule in each switching state is given by the sum of the transmission voltages of two semiconductors, through which the full terminal current ix flows.
It is therefore evident that, both in terms of the number of semiconductor switches to be installed and in terms of the power loss, the full-bridge submodule of FIGS. 3 to 5 is less favourable than the half-bridge submodule of FIG. 2. However, these disadvantages are countered by the following advantages of modular multi-level converters with full-bridge-submodules:    a) the DC current at the DC terminals of the converter can be electronically switched off, for example in the event of DC short-circuits,    b) the DC voltage at the DC terminals can be continuously adjusted between a positive maximum value and a negative maximum value of equal amplitude, independently of the AC voltage, and    c) the capacitance of the capacitors in the submodules can be dimensioned significantly smaller, because the power polarization is inherently lower.
As a kind of intermediate solution between the half-bridge and full-bridge submodule, a submodule has been proposed in DE 10 2009 057 288, which is reproduced in FIG. 6. The submodule of FIG. 6 also offers the advantage mentioned in (a) above, that the DC current can be electronically switched off, and in comparison to converters with full-bridge submodules, allows a reduction in the power loss of up to 25%. However, the advantages listed under (b) and (c) above cannot be achieved. A further restriction exists with regard to the maximum value of the negative DC voltage, which can only reach half the value compared with that of the full-bridge submodules.
An extension to the submodules known from DE 10 2009 057 288 A1 and shown in FIG. 6 by two additional semiconductor switches is specified in the following doctoral thesis from the Swedish Research Institute KTH: K. Ilves, “Modeling and Design of Modular Multilevel Converters for Grid Applications”, Doctoral thesis, KTH Royal Institute of Technology, Stockholm, Sweden, TRITA-EE 2014:045, ISSN 1653-5146, urn:nbn;se:kth:diva-153762.
The version designated in the aforementioned thesis as a “semi-fullbridge submodule” essentially enables criterion (b) to be satisfied. But there is still a restriction on the attainable maximum value of the negative DC voltage, which continues to reach only the value according to DE 10 2009 057 288 A1 and FIG. 6. Furthermore, there exist disadvantages with regard to criterion c), because switching states with direct parallel connection of the two storage capacitors present in the same submodule are permitted only to a very limited extent. At high voltages, a direct parallel connection of capacitors, as is known to the person skilled in the art, leads to short-circuit-like DC compensation currents and inherent energy losses.
DE 10 2013 205 562 discloses an energy storage device which is designed to deliver and/or absorb electrical energy in the form of an n-phase current and an n-phase voltage, where n>1. The energy storage device comprises n energy supply branches, each of the energy supply branches having a plurality of energy storage modules connected in series. The energy storage modules each comprise an energy storage cell coupling module with coupling module connectors, and a coupling device with coupling elements, which are designed to selectively switch the energy storage cell module via the coupling module connectors into the respective energy supply branch or to bypass the respective power supply branch. Each of the energy storage cell coupling modules in turn has a coupling module strand with a plurality of series-connected energy storage cell branch modules, which comprise an energy storage cell branch with a series circuit comprising an energy storage cell branch coupling element and at least one energy storage cell, and a bypass-branch coupling element connected in parallel with the energy storage cell branch.